a real world point of view
On-line conform or on-line editing — generally unhyphenated in American English — is the final stage of the editing process. An on-line editor starts with the locked edit provided by the editor. He or she will then add in visual effects shots, grading improvements by the colourist, the final audio mix and other post-production improvements to create the edit of the episode actually broadcast. The on-line editor will also ensure that the entire programme meets — or conforms to — certain technical specifications like the aspect ratio. Because of these duties, the on-line editor is essentially the last person to work on an episode of Doctor Who.
Unlike an editor, the on-line editor's job is not about improving the narrative "flow" of the story, but rather about the purely technical work of assembling and adding technical improvements to the raw footage that the editor has assembled off-line. It derives its name, "on-line", from the fact that the work flow of finishing the episode is done over a computer network which links several different people together in the final push to finish an episode.
People providing this service were generally credited as "on-line editors" during the Russell T Davies era of the programme. The term "on-line conform" was first used on The Eleventh Hour, and refers to the actual name of this edit. The "conform" is the edit which conforms to the British Broadcasting Corporation's technical standards for its television programmes.